Youtube, for me has always had video playing and a scrollable list of other clips to the right, meaning you can check a high number of alternatives without losing sight of the video playing and there is the option to go 'cinema mode' or full screen.

I updated to Firefox 57 (Quantum) last week and found the video has a bigger area and alternative videos are listed below. Still an option to go cinema mode or full sccreen but if you want to see a suggested video list you lose sight of the one playing.

Due to a number of other issues with FF57 I went back to a previous version and Youtube is running as I'm used to.

It has gone from what was basically a horizontal scrolling screen to a vertical one that doesn't "stay" within the boundaries of the screen.

It is hard on the eyes!

I can imagine. You'd think it would be quite simple for any given device to "know" its screen size and default to an appropriate or universal fit, allowing users to tinker as they want. NoSquint Plus or an equivalent ought to be a feature of every browser, not an add-on that then doesn't work because of an update.